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== Phase trajectory ==
[[Minimum-shift keying]] (MSK) is another name for CPM with an excess bandwidth of 1/2 and a linear ''phase trajectory''. Although this linear phase trajectory is continuous, it is not ''smooth'' since the derivative of the phase is not continuous. The spectral efficiency of CPM can be further improved by using a smooth phase trajectory. This is typically accomplished by filtering the phase trajectory prior to modulation, commonly using a [[raised cosine]]
or a [[Gaussian filter]]. The raised cosine filter has zero crossings offset by exactly one symbol time, and so it can yield a ''full-response'' CPM waveform that prevents [[intersymbol interference]] (ISI).
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